An elevator is a hoisting and lowering mechanism equipped with a car or platform which moves in guides in a vertical direction. Traction elevator systems typically include a cab, a counterweight, one or more ropes interconnecting the cab and counterweight, a traction sheave to move the rope(s), and a motor to rotate the traction sheave. Elevator ropes conventionally comprise laid or twisted steel wire and the sheave is formed of cast iron.
Electric elevators are suspended and moved by a series of pulleys (sheaves) and cables (ropes). In a typical arrangement, a wire rope is reeved over a number of sheaves, terminating in a hitch on the top and bottom of the car. Care is typically taken during the installation to mount the various sheaves in the machine room or on the car in such a manner as to ensure only vertical strain is applied to the wire ropes terminating in hitches. This reduces metal fatigue on the wire rope that would occur if subjected to horizontal bending forces.
Tensile strength measures the stress required to pull something such as rope, wire, or a structural beam to the point where it breaks. It is an intensive property of the material. The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. There are three typical definitions of tensile strength: (1) yield strength, which is the stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently; (2) ultimate strength, which is the maximum stress a material can withstand; and (3) breaking strength, which is the stress coordinate on the stress-strain curve at the point of rupture. The tensile strength of an elevator rope system is often at its weakest point at the termination of the elevator rope. The uniform forces on the rope along the wedge effectively lock the rope in the termination. As the rope tension increases, the differential compressive forces at the nose of the termination will also increase accordingly.
For example, a rope having a rated breaking strength of 25,000 lbs may be limited by the termination device, which may reduce the overall breaking strength of the system to a fraction of the percentage, such as 65% of the rope itself. According to the European code (EN81), the minimum required breaking strength of the termination is 85% of the full breaking strength of the rope. If the rope termination is only 65% of the breaking strength of the rope, the rope will have to be rated at a lower number to meet this requirement.
Coated steel belts (CSB's) have been developed which are strong enough to replace the traditional wire cables used with elevators. These CSB's permit sheave and hitch arrangements that were not practical when using wire ropes. In an arrangement where the CSB hitch or termination is subjected to horizontal forces as well as vertical forces, metal fatigue is a concern. Conventional steel ropes and the cast iron sheaves that move them have certain limitations in their use. One such limitation is the traction forces between the ropes and the sheave. Drive sheaves with large diameters are often needed to obtain the required traction to move the components in the system without the rope slipping over the sheave. Another limitation on the use of steel ropes is the flexibility and fatigue characteristics of steel wire ropes. Aramid-based ropes are being developed to overcome the problems associated with steel cables. Conventional termination devices, however, do not readily lend themselves to use with aramid-based ropes, where aramid-based ropes tend to slip out of such devices and, consequently, a dangerous condition may result. Furthermore, termination devices may reduce the overall breaking strength of the elevator rope system such that more rope will be needed to meet the minimum safety factor.
Accordingly, there is a need for a termination device that can accommodate aramid-based elevator ropes. It would be further advantageous to provide a termination device to accommodate aramid-based elevator ropes that minimally reduces the breaking strength of the overall system. It would be further advantageous to provide a termination device that reduces the breaking strength of a rope by 85% or less to avoid such that the rope will no longer need to be rated at a lower breaking strength and less rope can be used for a specific application.
Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.